What does a parathyroid hormone (PTH) level of 12 indicate?

Interpretation of PTH Level of 12

A PTH level of 12 pg/mL is significantly below the normal reference range and indicates hypoparathyroidism or suppressed parathyroid function, requiring immediate evaluation of serum calcium, phosphate, and vitamin D status to determine the underlying cause and guide management.

Understanding the PTH Value

• Normal PTH reference ranges typically fall between approximately 15-65 pg/mL (1.6-7.2 pmol/L), though this varies by assay generation and laboratory-specific reference values 1.
• A PTH of 12 pg/mL is below the lower limit of normal, indicating inadequate parathyroid hormone production or suppression 1.
• Assay-specific interpretation is critical because second-generation and third-generation PTH assays can yield measurements that vary up to 47%, and different antibody designs within the same generation can produce different results 1.

Clinical Context and Differential Diagnosis

Primary Considerations

Hypoparathyroidism is the most likely diagnosis when PTH is low:

• Post-surgical hypoparathyroidism following thyroid or parathyroid surgery is the most common cause, where parathyroid glands are damaged or inadvertently removed 1.
• Autoimmune hypoparathyroidism or genetic causes should be considered in the absence of surgical history 1.

Suppressed PTH from hypercalcemia must be ruled out:

• Measure serum calcium immediately - if calcium is elevated, PTH suppression is physiologically appropriate 2, 3.
• Causes of hypercalcemia with low PTH include malignancy, hypervitaminosis D, granulomatous disease, or immobilization 3, 4.

Essential Laboratory Workup

Immediate measurements needed:

• Serum calcium (corrected for albumin or ionized calcium) to determine if hypocalcemia or hypercalcemia is present 2, 4.
• Serum phosphate - expect elevated phosphate in hypoparathyroidism due to loss of PTH's phosphaturic effect 2.
• Serum creatinine and eGFR to assess kidney function, as renal disease affects PTH interpretation 2.
• 25-hydroxyvitamin D level because vitamin D deficiency can complicate the clinical picture 2, 1.
• Serum magnesium as severe hypomagnesemia can suppress PTH secretion 1.

Assay-Related Considerations

Critical technical factors:

• Use assay-specific reference values rather than generic cutoffs, as PTH measurements vary significantly between laboratories and assay generations 1, 2.
• Second-generation assays measure intact PTH plus the 7-84 fragment, while third-generation assays measure only full-length PTH (1-84), leading to different absolute values 1.
• Biotin supplementation can interfere with PTH assays, causing either overestimation or underestimation depending on assay design 1.
• PTH is most stable when measured in EDTA plasma rather than serum 1, 2.

Factors Influencing PTH Levels

Biological variables to consider:

• Age: PTH increases with age due to declining GFR 1.
• Race: PTH concentrations are higher in Black individuals compared to White individuals 1.
• BMI: PTH is higher in obese patients 1.
• Vitamin D status: Deficiency causes secondary elevation of PTH, while repletion normalizes levels 1.

Common Pitfalls to Avoid

• Do not interpret PTH in isolation - always correlate with serum calcium, phosphate, and kidney function 2, 4.
• Avoid using absolute cutoff values from guidelines without considering the specific PTH assay used in your laboratory, as guidelines often do not specify which assay generation their recommendations are based on 1.
• Do not assume secondary hyperparathyroidism in chronic kidney disease patients with low PTH - adynamic bone disease is characterized by PTH <100 pg/mL (11.0 pmol/L) and requires different management 1.
• Check medication history for drugs that can suppress PTH or affect calcium metabolism 3.
• Verify the patient is not taking biotin supplements before interpreting results, as this can cause spurious values 1, 2.

Clinical Implications by Calcium Status

If hypocalcemic (most likely scenario):

• Confirms hypoparathyroidism requiring calcium and active vitamin D supplementation 5.
• Investigate timing relative to any neck surgery 1.
• Consider genetic testing if no clear acquired cause 1.

If normocalcemic:

• May represent early or compensated hypoparathyroidism 6.
• Assess for adequate calcium and vitamin D intake masking the condition 5.

If hypercalcemic:

• PTH suppression is appropriate - investigate non-PTH-mediated causes of hypercalcemia 3, 4.
• Consider malignancy, vitamin D toxicity, or granulomatous disease 3.